In recent years, various flat-panel displays have been developed. Organic electroluminescent display devices (hereinafter referred to as “organic EL display device”) including an organic electroluminescent element (hereinafter referred to as “organic EL element”), especially, are attracting much attention as excellent flat-panel displays as these devices can achieve power saving, thickness reduction, improved image quality, and the like.
The organic EL element included in an organic EL display device has a configuration in which a luminescent layer containing a luminescent material made of an organic compound is sandwiched between a cathode and an anode. The organic EL element emits light by utilizing a mechanism in which electrons and positive holes (holes) are introduced into the luminescent layer, the electrons and positive holes are caused to recombine so that excitons are generated thereby, and light is emitted when the excitons lose their activity.
Such a luminescent material contains organic molecules that are excited from the ground state (S0 state) to an excited state as electrons at highest occupied molecular orbital (HOMO) levels absorb energy to transition to lowest unoccupied molecular orbital (LUMO) levels.
Organic molecules have two excited states that differ from each other in spin multiplicity, namely (i) a singlet excited state (S1 state), in which the respective spinning directions of electrons at HOMO levels and those at LUMO levels are antiparallel to each other, and (ii) a triplet excited state (T1 state), in which the respective spinning directions of electrons at HOMO levels and those at LUMO levels are parallel to each other.
As described above, the organic EL element utilizes a mechanism in which electrons and positive holes (holes) are introduced into the luminescent layer, the electrons and positive holes are caused to recombine so that excitons are generated thereby, and light is emitted when the excitons lose their activity. Such an exciton generated has a 25% chance of having the S1 state and a 75% chance of having the T1 state. When the exciton transitions from the triplet excited state (T1 state) to the ground state (S0 state), the exciton typically emits phosphorescence over a very long light emission lifetime of not less than several milliseconds and has a rate constant for heat inactivation which rate constant is larger than the rate constant for phosphorescence emission. The exciton is known to usually undergo radiationless deactivation, that is, to emit no light and release heat, at room temperature. Thus, a typical fluorescent material is said to have an internal quantum yield with a limit of 25%.
There is currently a phosphorescent material containing, as a central metal, a heavy atom such as an iridium complex. It has been made possible for such a phosphorescent material to (i) allow excitons having a singlet state to transition to a triplet state through exchange crossing as a result of an internal heavy atom effect and thereby (ii) cause all the singlet excitons and triplet excitons to contribute to light emission.
Recent years have seen development of a thermally activated delayed-emission material (hereinafter referred to as “TADF material”), which has an extremely small energy difference between an energy level (hereinafter referred to as “ES1 level”) in the lowest singlet excited state (S1 state) and an energy level (hereinafter referred to as “ET1 level”) in the lowest triplet excited state (T1 state). For a material that emits blue light, in particular, there has been development of new luminescent materials having a high light emission efficiency and a short-wavelength light emission peak.
A TADF material has an extremely small energy difference ΔEST between the ES1 level and the ET1 level, and allows excitons in the T1 state to become excitons in the S1 state due to inverse intersystem crossing. A TADF material thus allows excitons to contribute to light emission at 100%.
There has recently been disclosed a technique of combining a phosphorescent material such as the above with a fluorescent material to transfer energy to a target component for light emission by that component, thereby allowing a light-emitting element to have a longer life and a higher light emission efficiency.
Patent Literature 1, for example, discloses an organic EL element including (i) a first light-emitting layer containing at least one phosphorescence emitter and (ii) a second light-emitting layer adjacent to the first light-emitting layer and containing a fluorescence emitter that emits light mainly within a spectral range on the shorter wavelength side of the light emission peak of the first light-emitting layer. The fluorescence emitter of the second light-emitting layer is a substance having delayed fluorescence as a result of (i) energy transfer of triplet excitons from at least one phosphorescence emitter in the first light-emitting layer and (ii) upconversion from the triplet excited state to the singlet excited state. This makes it possible to provide an organic EL light-emitting device that is capable of emitting a plurality of colors of light including white light and that has a longer life.
Non-Patent Literature 1 discloses a light emission mechanism that involves a crystalline material having a metal-organic framework (MOF) and containing acceptor molecules self-organized in a regular pattern and that enables photon upconversion in a solid having a MOF.